Lithium batteries are used as power sources for portable phones, notebook PCs, etc. Thus, the lithium batteries are important devices which support advanced information society of today. Power consumption in such portable electronic devices shows a significant increase due to increase in information processing amount the portable electronic devices should deal with. Accordingly, there is a persistent demand for greater energy density of lithium batteries which are power sources of the portable electronic devices.
On the other hand, realization of environment-friendly society is a global-scale urgent issue. For this purpose, more efficient energy utilization and use of renewable energy have been carried on. One of such measures is use of electric vehicles. However, it is said that an energy density several times higher than that of an existing lithium-ion battery is required in order that an electric vehicle is realized. Accordingly, increasing energy densities of lithium batteries is an important issue in such a field too.
A lithium-ion battery which is a most prevalent lithium battery is a combination of a graphite negative electrode and a LiCoO2 positive electrode. As long as this combination is employed, it is difficult to further increase energy density from the current level. In order to meet the aforementioned social demands, there is an urgent need for development of a new electrode material having an electrical capacity higher than those of such conventional electrode materials.
Candidate materials for a high-electrical capacity negative electrode of a lithium battery have been already known by learning their electrochemical equivalents which are calculated from (i) how much atomic weight or molecular weight the materials have and (ii) how many electrons participate in an electrode reaction. Lithium metal, a lithium alloy, etc. are such conventionally known candidate materials. Other than such classically-known negative-electrode materials, Tarascon et al, proposed a conversion electrode which is a high-electrical capacity negative electrode based on a new concept (see Non-patent Literature 1).
The conversion reaction is such a reaction that a metal oxide such as CoO or NiO is electrochemically reduced in a lithium battery so that metal fine particles such as cobalt fine particles or nickel fine particles are generated from the metal oxide.
The conversion reaction is reversible. For example, in a case where a metal atom is cobalt, the conversion reaction is as follows, for example: CoO+2e−+2Li+Co+Li2O; or Co3O4+8e−+8Li+3Co+4Li2O. These reactions respectively yields such great capacities of 715 mAh·g−1 and 891 mAh·g−1.
Metal elements disclosed in Non-Patent Literature 1 are cobalt, nickel, copper, and iron only. If a metal element which can be alloyed with lithium is employed instead of these metal elements, an alloying reaction follows the conversion reaction in the reduction reaction in the lithium battery. It is expectable that this configuration can yield a further great electrical capacity. Non-patent Literature 2 proposes to produce the negative electrode of the lithium battery from tin oxide as the metal oxide which forms a lithium alloy as described above. In this case, reactions which repeatedly take place are the alloying reaction and the dealloying reaction only, and the conversion reaction occurs only in a first reduction process. This phenomenon observed in the conversion reaction of an oxide can be observed in a conversion reaction of a sulfide. In a first reduction process of SnS2, a generation reaction of tin as a simple substance is observed at 0.8 V with reference to a lithium electrode. However, a re-oxidation reaction corresponding to the reduction reaction is not observed (Non-patent Literature 3). That is, although it is conventionally known that the combination of the conversion reaction and the alloying reaction would possibly serve as a high-electrical capacity negative electrode reaction, there has been no report that these reactions are repeatedly caused to realize such a negative electrode reaction with a high electrical capacity density.
An electrical capacity of the conversion reaction yields is determined by how many electrons take part in the reaction converting a compound to a metal. An electrical capacity of the alloying reaction is determined by how much lithium in the composition is available for the formation of the lithium alloy. A highest electrical capacity density can be expected in a case where a silicon compound is employed. However, there has been no proposal of a negative electrode utilizing (i) a conversion reaction of a silicide and (ii) a subsequent alloying reaction with lithium.
The inventors of the present invention found that the reason why the combination of the conversion reaction of a sulfide of silicon and the subsequent alloying reaction with lithium is not employed as the negative electrode reaction in the lithium battery is an extremely poor repeatability of the conversion reaction and the alloying reaction.
This is explained below, referring to the case of an electrode reaction of silicon sulfide for example. In this case, a charge reaction of the negative electrode made from silicon sulfide is a reduction process starting from silicon sulfide. In this process, silicon sulfide is converted to an elemental silicon by the conversion reaction, and further converted to a lithium-silicon alloy by the alloying reaction. In a following discharging of the lithium battery, the lithium-silicon alloy generated in the charging process is supposed to be re-oxidized to be converted back to the elemental silicon, and further to silicon sulfide. However, although the generation reaction of elemental silicon and the generation reaction of the lithium-silicon alloy take place in the first reduction process, only the dealloying reaction take place in the subsequent re-oxidation process. Furthermore, a coulombic efficiency is extremely low which is a ratio of an electrical capacity in the first reduction process to an electrical capacity in the first re-oxidation process (hereinafter, a coulombic efficiency for the first reduction process and the first re-oxidation process is referred to as first-cycle coulombic efficiency). The coulombic efficiency is not sufficient for a high-electrical capacity negative electrode of a rechargeable lithium secondary battery.
Citation List
Non-Patent Literatures
Non-Patent Literature 1
P. Poizot, S. Laruelle, S. Grugeon, L. DUPONT and J.-M. Tarascon, Nature 407, pp. 496-499 (2000).
Non-Patent Literature 2
Y. Idota, T. Kubata, A. Matsufuji, Y. Maekawa and T. Miyasaka, Science, 276, pp. 1395-1397 (1997).
Non-Patent Literature 3
T. Brousse, S. M. Lee, L. Pasquereau, D. Defives and D. M. Schleichi, Solid State Ionics, 113-115, pp. 51-56 (1998).